Hard disc drives are used in modern computer systems to enable users to store and retrieve vast amounts of data in a fast and efficient manner.
In a typical disc drive, one or more magnetic discs are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs on air bearings established by air currents set up by the rotation of the discs. Each head includes a write element that selectively magnetizes data fields defined on tracks on the corresponding disc surface during a write operation, and a read element that detects the selective magnetization of the data fields during a read operation. A read/write channel and an interface circuit, responsive to the heads, are provided to transfer the data between the discs and a host computer in which the disc drive is mounted.
A closed loop digital servo system is used to control the position of the heads relative to the tracks through the application of current to a coil of a voice coil motor. The tracks are defined from servo information written to servo fields on the surfaces of the discs during manufacturing using a highly precise servo track writer. The servo information is stored in a series of servo fields, the leading edges of which are radially aligned on each of the surfaces of the discs so as to define servo wedges which outwardly extend from the inner radii of the discs like spokes of a wheel. The data fields are subsequently arranged between adjacent servo fields during a disc drive formatting operation. Typical disc drives generally provide from about 30 to 90 servo wedges on each disc surface.
The servo information typically includes automatic gain control (AGC), synchronization, track address, radial position (index) and position information stored in associated fields, with the AGC and synchronization information providing timing and amplitude inputs, the track address information indicating the radial position of the heads, the index information indicating the angular position of the heads with respect to the discs and the position information indicating the position of the heads with respect to the center associated tracks on the discs. Thus, during normal disc drive operation, the servo information is periodically sampled to enable the servo system to control the position of the heads to properly effectuate the transfer of data between the data fields and the host computer.
The frequency at which the data are written to the data fields is selected to be as high as practicable in order to maximize the data transfer characteristics of the disc drive. The use of magneto-resistive (MR) heads and partial response, maximum likelihood (PRML) read channel detection techniques have allowed disc drives of the present generation to write and read data at frequencies of up to 200 megahertz (MHZ). Typically, however, the servo information is written at a substantially lower frequency, such as 20 MHZ. This reduction in the frequency at which servo information is written is due to a variety of considerations, including the fact that the retrieval of data is accomplished primarily through detecting the presence (or absence) of flux transition pulses from the media; compensation for the effects of factors such as noise and intersymbol interference can hence be sufficiently employed to decode the data, even when the readback pulses are adjacently disposed. By contrast, proper operation of the servo system requires accurate determination of pulse location and amplitude of the servo information which is typically difficult to accurately detect in the presence of noise and interference characteristic of information written at the higher frequencies used to store and retrieve the user data.
Most prior art disc drives have servo information that is written at the same frequency for each of the tracks on the discs. As mentioned above, this results in the servo information being arranged as a plurality of wedges that are wider at the outer radii as compared to the inner radii of the discs (i.e., servo fields written at the outer radii are physically larger than servo fields at the inner radii of the discs). Although such an approach is relatively easy to implement and control, the optimum frequency for the recovery of the servo information by the servo system is not necessarily constant, but rather varies in relation to the radii of the discs.
Generally, the optimum frequency for servo recovery is determined by the pulse width characteristics of the head (such as, for example, PW50, which is a measure of pulse width with respect to amplitude) and the magnetic grain size of the media. At too high a frequency the recording characteristics of the media become non-linear which adversely increases noise in the servo readback signal; at too low a frequency the pulses are too narrow, resulting in a low signal to noise ratio. Thus, the constant frequency at which servo information is written is usually selected to ensure adequate readback performance over the entire radii of the discs and to accommodate other factors, such as alternating current (AC) coupling characteristics of the heads.
Some prior art disc drives, have been disclosed having servo information arranged in a plurality of radially defined zones on the discs, each zone comprising a selected band of tracks to which user data are stored in data fields ("sectors") of substantially equal circumferential length. Examples of such prior art disc drives are provided in, for example, U.S. Pat. No. 4,016,603 issued Apr. 5, 1977 to Ottesen (hereinafter "Ottesen '603") and U.S. Pat. No. 5,193,034 issued Mar. 9, 1993 to Tsuyoshi et al. (hereinafter "Tsuyoshi '034").
Ottesen '603, a relatively early disc drive patent, arranges the servo information into a different number of constant-frequency servo wedges in each of the zones, with the servo wedges in one zone radially offset from the servo wedges in the next, adjacent zone. Although operable, Ottesen '603 has a number of associated problems, including relative difficulty in ensuring proper write protection for each of the servo wedges (which are scattered over the surfaces of the discs), the requirement for an external index generator so that absolute angular position can be determined and maintained (Ottesen '603employs a tachometer), and the potential for loss of servo control when a selected head is moved to adjacent tracks disposed on either side of a selected zone boundary.
Tsuyoshi '034 also discloses zone based recording, but radially aligns the servo information so as to provide continuously extending servo wedges across the surfaces of the discs, as in the other prior art disc drives discussed above. Particularly, the servo information is written as a plurality of radially extending wedges across each of the zones, the wedges having increased widths at the outer diameters of the discs. While Tsuyoshi '034 extends the servo information across the boundary regions between adjacent zones in order to address the zone boundary control problems associated with Ottesen '603, and writes some of the servo information at a different frequency (on a zone basis) in order to subsequently optimize the writing of the user data to the sectors in each zone, Tsuyoshi '034 still requires substantially the same disc surface overhead for the servo information as is common in the prior art.
Accordingly, as data storage densities continue to increase, there remains a continual need for improvements in the art whereby greater levels of disc drive servo performance and data storage capacities can be achieved.